Apart from the basic difference in meaning between prescribed and dégagé, the ECJ cannot "prescribe" anything to a French court unless the case involves EU law (as opposed to French national law). Although you're of course right that the ECJ, like any court, has enforcement powers, it only has them with respect to EU law:
"The ECJ is the highest court of the European Union in matters of Union law, but not national law. It is not possible to appeal against the decisions of national courts in the ECJ, but rather national courts refer questions of EU law to the ECJ."
https://en.wikipedia.org/wiki/European_Court_of_Justice It is very similar to the difference between federal and state courts in the US.
@ AllegroTrans: if this WERE a question of EU law, or if the method of evaluating trademarks had been held to be the correct method by the FR Cour de cassation, then a method of evaluating trademarks absolutely could be "prescribed" by a higher court. That would mean the higher court is telling lower courts that if they're evaluating trademarks, they have to do it this way (following this method). That just doesn't happen to be the case here (nor does "dégagé" mean "prescribed").